The present invention concerns a method for operating an incandescent lamp with a DC voltage and, in particular, a method of operating an incandescent lamp as a source of illumination in an electro-optical measurement device.
Incandescent lamps are commonly operated either with a DC voltage or with a sinusoidally alternating current; however, investigations have shown that when operating temperatures of the lamp filament are below 2,700.degree. K., as is nearly always the case in miniature lamps, for example, lamps operated with DC voltage fail at a substantially higher rate than do lamps operated with alternating current. Scanning electron microscope pictures of incandescent lamp filament have shown that when an alternating current is used to operate a lamp the lamp filament have little or no tendency to form bevels on the outside while when a DC voltage is used a noted beveling takes place on both the outside and inside surfaces of the lamp filament. Generally, the bevels on the inside and the outside of the filament are not parallel to each other, and the resulting constrictions often develop into deep notches which weaken the filament and lead to its premature breakage. Because of this beveling phenomenon, miniature lamps are preferably operated with a sinusoidally alternating current in most applications.
In many electro-optical measurement devices, however, lamps should not be operated with a sinusoidally alternating current, for such operation results in modulations of the lamp luminosity and the measurement signal which cannot be detected by the eye but which result in measurement errors.
Furthermore, there are known methods for operating lamps in an impulse manner to increase lamp performance without influencing the life of the lamp, specifically to increase the life of the lamp at a given performance (West German DT-AS No. 1 589 223, West German DT-OS No. 1 539 453). In this mode of operation the lamp operating voltage may increase from zero to a predetermined value V and then, after a specific time, drop again to zero.
Furthermore, there are known procedures for regulating and stabilizing lamp output in which a lamp is operated with square wave impulses having an adjustable width (West German DT-OS No. 2 423 153, West German DT-OS No. 1 489 383, West German DT-PS No. 1 589 120).